FIG. 11 is a block circuit diagram of a first conventional driving apparatus including an inverter for driving a multiple induction motors, not provided with any rotating speed detector. In FIG. 11, N represents the number of the induction motors. The first conventional driving apparatus includes an inverter 1, current detectors 21, 22, and 23 that detect the output currents from the inverter 1, induction motors 31 through 3N, and a controller 11. The controller 11 includes a rotating speed estimator 40, a trans-vector control calculator 41, a torque regulator 110, and a subtractor 45.
The controller 11 works in the following manner. The rotating speed estimator 40 estimates the rotating speed based on the three-phase reference voltages vU*, vV*, vW* (hereinafter referred to as “primary reference voltages”) outputted from the trans-vector control calculator circuit 41 and the inverter output currents iU, iV, iW detected by the current detectors 21, 22, 23. The estimated rotating speed ω# outputted from the rotating speed estimator 40, the second reference torque τ** outputted from the subtractor 45, the reference magnetic flux φ*, and the inverter output currents iU, iV, iW detected by the current detectors 21, 22, 23 are inputted to the trans-vector control calculator circuit 41. The trans-vector control calculator circuit 41 conducts trans-vector control calculation based on the inputs described above and outputs primary reference voltages vU*, vV*, and vW*.
The torque regulator circuit 110 judges that the rotating speed differences have widened when the time variation rate of the estimated rotating speed ω# outputted from the rotating speed estimator 40 exceeds a specified value and outputs a torque reduction quantity Δτ. The subtractor 45 subtracts the torque reduction quantity Δτ outputted from the torque regulator circuit 110 from first reference torque τ* and outputs second reference torque τ**. The rotating speed differences are prevented from widening as described above.
The rotating speed, which the rotating speed estimator 40 can estimate, is the average rotating speed of N motors. In the case of a driving apparatus for driving a railway car, very often one inverter drives four or more motors. Therefore, even when the rotating speed differences are widened due to the rotating speed up or the rotating speed down of one motor, the rotating speed difference widening will not be detected, if none of the rotating speed differences exceeds a certain limit to the wider side.
Japanese patent publication JP P 2002-281606 A (pp. 5–6, FIG. 1) discloses a second conventional driving apparatus for obviating the problem described above. FIG. 12 is a block circuit diagram of the second conventional driving apparatus shown in FIG. 1 of JP P 2002-281606 A. In FIG. 12, the second conventional driving apparatus drives two motors exemplary (N=2). The second conventional driving apparatus shown in FIG. 12 provides a motor 31 with current detectors 24, 25, 26 and a motor 32 with current detectors 27, 28, 29 and includes a slip/slide detector 121 that detects the rotating speed difference widening based on the relative difference of each motor current.
In detail, a coordinate transforming circuit 120 transforms the three-phase motor currents to two-phase quantities on the rotatory coordinate axes (dq-axes) used in trans-vector control according to the following formulas (1), (2) and calculates the q-axis currents i1q1 and i1g2 for the respective motors. In the formulas (1) and (2), θ is a reference phase angle calculated in the trans-vector control calculator circuit 41 and used for the trans-vector control calculations.
                                                                                                    i                                      1                    ⁢                    d                    ⁢                                                                                  ⁢                    1                                                                                                                        i                                      1                    ⁢                    q                    ⁢                                                                                  ⁢                    1                                                                                                =                                                                                                                                    2                      /                      3                                        ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                    θ                                                                                                                                                                -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                        +                                                                  1                        /                                                  3                                                                    ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                                                                                                                                                                    -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                        -                                          1                      ⁢                                              3                                            ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                                                                                                                                              2                      /                      3                                        ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    θ                                                                                                                                                                -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                        +                                                                  1                        /                                                  3                                                                    ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                                                                                    -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                        -                                                                  1                        /                                                  3                                                                    ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                ⁢                                                                                                                                                                              i                                                      U                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                                                        i                                                      V                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                                                                                i                                          W                      ⁢                                                                                          ⁢                      1                                                                                                                                          (        1        )                                                                                                                i                                      1                    ⁢                    d                    ⁢                                                                                  ⁢                    2                                                                                                                        i                                      1                    ⁢                    q                    ⁢                                                                                  ⁢                    2                                                                                                =                                                                                                                                    2                      /                      3                                        ⁢                                                                                  ⁢                    cos                    ⁢                                                                                  ⁢                    θ                                                                                                                                                                -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                        +                                                                  1                        /                                                  3                                                                    ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                                                                                                                                                                    -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                        -                                          1                      ⁢                                              3                                            ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                                                                                                                                              2                      /                      3                                        ⁢                                                                                  ⁢                    sin                    ⁢                                                                                  ⁢                    θ                                                                                                                                                                -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                        +                                                                  1                        /                                                  3                                                                    ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                                                                                    -                          1                                                /                        3                                            ⁢                                                                                          ⁢                      sin                      ⁢                                                                                          ⁢                      θ                                        -                                                                  1                        /                                                  3                                                                    ⁢                                                                                          ⁢                      cos                      ⁢                                                                                          ⁢                      θ                                                                                                                ⁢                                                                                                                                                                              i                                                      U                            ⁢                                                                                                                  ⁢                            2                                                                                                                                                                                        i                                                      V                            ⁢                                                                                                                  ⁢                            2                                                                                                                                                                                                                i                                          W                      ⁢                                                                                          ⁢                      2                                                                                                                                          (        2        )            
Although the q-axis currents i1q1 and i1g2 for the respective motors do not cause any difference as shown in FIGS. 13A–13C as far as the rotating speed deviations of the respective motors are small, the q-axis currents i1q1 and i1g2 cause a difference as shown in FIGS. 14A–14C when the rotating speed deviations of the respective motors are large. Considering these characteristics, the second conventional driving apparatus quickly detects, with the slip/slide detector 121, a motor rotating speed difference Δωs from the deviations of the q-axis currents i1q1 and i1g2 for the respective motors and reduces, before the rotating speed difference becomes wide, the torque by the torque reduction quantity Δτ calculated in a torque regulator circuit 122 to obviate the problem of the first conventional driving apparatus.
However, it is necessary for the second conventional driving apparatus shown in the JP P 2002-281606. A to provide every motor with current detectors. For driving a railway car by the second conventional driving apparatus, it is necessary to arrange the current detectors in the bogie of the car, since the power lines to the motors are divided in the bogie. Therefore, it is necessary for the current detectors to exhibits excellent resistance to vibration. In view of the foregoing, it would be desirable to provide a driving apparatus for driving multiple motors that makes it unnecessary to provide every motor with current detectors, and that facilitates quickly detecting the widening of the differences between the motor rotating speeds so that such widening may be prevented.